How to choose between SaaS and open source?

I choose to build a station, see there are a lot of options, just contact this choice is difficult, mainly do not understand, just how to choose, there is no know speak in detail?

SaaS building low threshold, simple and convenient, fast online, the cost seems to be okay so.
There are also open source builders WordPress, Magento, Joomla these open source builders are time consuming.